BAGHDAD, Sept. 27 — The radical Shiite cleric Moktada al-Sadr has lost control of portions of his Mahdi Army militia that are splintering off into freelance death squads and criminal gangs, a senior coalition intelligence official said Wednesday.

The question of how tightly Mr. Sadr holds the militia, one of the largest armed groups in Iraq, is of critical importance to American and Iraqi officials. Seeking to ease the sectarian violence raging across the country, they have pressed him to join the political process and curb his fighters, who see themselves as defenders of Shiism — and often as agents of vengeance against Sunnis.

But as Mr. Sadr has taken a more active role in the government, as many as a third of his militiamen have grown frustrated with the constraints of compromise and have broken off, often selling their services to the highest bidders, said the official, who spoke to reporters in Baghdad on condition of anonymity because he was not permitted to speak publicly on intelligence issues.

“When Sadr says you can’t do this, for whatever political reason, that’s when they start to go rogue,” the official said. “Frankly, at that point, they start to become very open to alternative sources of sponsorship.” The official said that opened the door to control by Iran.

Mr. Sadr’s militia — dominated by impoverished Shiites who are loosely organized into groups that resemble neighborhood protection forces — has always operated in a grass-roots style but generally tended to heed his commands. It answered his call to battle American forces in two uprisings in 2004, and stopped fighting when he ordered it. But as the violence in Iraq has spread, evidence of freelancing Shiites has accumulated.

After the bombing of a Shiite shrine in Samarra in February, bands of militants dressed in black, the favorite color of Sadr loyalists, drove into neighborhoods, kidnapping and killing Sunnis. Mr. Sadr, who was abroad at the time, returned home and gave a rare public speech calling on his followers to stop, even proposing joint prayer sessions with Sunni clerics. Still, the rampage continued.

In Basra, a province in southeastern Iraq, Mr. Sadr has less direct control over militiamen, and they have tended to operate to suit their own agenda. Local leaders there have said that he has disciplined some members and fired others, but with little overall effect. He has run through four different leaders in Basra, according to the intelligence official, and has even had to shut offices temporarily, when local leaders ignored him and acted on their own.

Mr. Sadr is still immensely powerful, with as many as 7,000 militiamen in Baghdad, the official said. And the cleric has turned that firepower into political might. His candidate list won about 30 seats in Parliament this year, one of the largest shares. The participation was a central goal for American officials, who tried for months to persuade him to stop fighting and enter politics.

Still, six major leaders here no longer answer to Mr. Sadr’s organization, according to the intelligence official. Most describe themselves as Mahdi Army members, the official said, and even get money from Mr. Sadr’s organization, but “are effectively beyond his control.” Some of those who moved away from Mr. Sadr saw him as too accommodating to the United States. Others saw him as too bound by politics, particularly as killings of Shiite civilians in mixed neighborhoods began to soar.

Rogue Sadrists providing cover for criminals and killing as they feel.